In a mobile communication system including multiple cells, a mobile station UE (User Equipment: user device) is configured to continue communication by switching from the current cell to another cell when the mobile station moves from the current cell to the other cell. Such switching between cells is called “handover.”
Generally, in a mobile communication system, when the mobile station UE moves to an adjacent cell, and the received power of signals from the adjacent cell at the mobile station UE becomes greater than the received power of signals from a Serving Cell, the mobile station UE performs a handover to the adjacent cell.
The received power of signals from the above-mentioned adjacent cell or the received power of signals from the serving cell is, for example, downlink RSRP (Reference Signal Received power) of downlink reference signals transmitted from the adjacent cell or the serving cell.
An example of the procedure of such handover is specifically described with reference to FIGS. 5 and 6.
As shown in FIG. 5, in step S1, the mobile station UE measures the received power of signals from the adjacent cell.
In step S2, the mobile station UE determines whether or not the received power of signals from the adjacent cell satisfies the following formula (Formula 1).The received power of signals from the adjacent cell+hysteresis>the received power of signals from the serving cell  (Formula 1)
When it is determined that (Formula 1) is satisfied, in step S2, the mobile station UE notifies the network of an event A3 for reporting the above-mentioned measurement result.
Specifically, as shown in FIG. 6, the mobile station UE measures the received power of signals from the serving cell (cell A) and a peripheral cell (cell B) which are objects to be monitored, and determines whether or not the above-mentioned measurement result has to be notified by using “hysteresis [dB]” and “TTT (Time To Trigger) [ms]” which have been notified in advance.
That is to say, in FIG. 6, when the state in which the received power (radio quality) of signals from the cell B is greater than the received power (radio quality) of signals from the cell A by the “hysteresis” or more, continues for a predetermined time period of “TTT” or longer, the mobile station UE determines that the above-mentioned measurement result (Measurement Report) has to be notified.
As the received power (radio quality) of signals Fn, the values calculated by the following (Formula 2) and (Formula 3) are used. Specifically, in the mobile station UE, an upper layer is configured to perform the filtering processing (L3 Filtering) shown in (Formula 2) on the values measured by a physical layer.Fn=(1−a)·Fn−1+a·Mn  (Formula 2)a=½(k/4)  (Formula 3)
where the value of “k” in (Formula 3) has been notified from the radio base station to the mobile station UE in advance like the “hysteresis” and “TTT.” The “hysteresis” is a value provided to suppress frequent handover from the serving cell to the adjacent cell on the cell boundary, and may be a positive or negative value.
Further, in step S3, the network, when receiving a notification of the event A3, determines that the mobile station UE performs a handover to the cell according to the received event A3.
In a mobile communications system of the LTE (Long Term Evolution) scheme, DRX (Discontinuous Reception) control is applied.
Such discontinuous reception control is applied when the radio base station eNB and the mobile station UE are connected, and the data to be transmitted does not exist, and the mobile station UE in a discontinuous reception state is configured to receive a down control signal which is transmitted periodically, i.e., discontinuously via a PDCCH (Physical Downlink Control Channel).
In such a case, the mobile station UE only needs to receive discontinuously rather than continuously the down control signal transmitted via the physical downlink control channel PDCCH, thus the power consumption of a battery can be reduced.
More specifically, as shown in FIG. 7, the mobile station UE is configured to receive the down control signal transmitted via the physical downlink control channel PDCCH only in the reception duration (5 ms in the example of FIG. 7) provided every DRX period (1280 ms in the example of FIG. 7), and to turn other transmitters/receivers off. As a result, the power consumption of a battery can be reduced in the mobile station UE.
As described above, the mobile communication system of the LTE scheme controls whether each mobile station UE should be set in a discontinuous reception state or continuous reception state based on the existence of data to be transmitted, the above-described handover is performed regardless of whether or not the mobile station UE is in a discontinuous reception state.